Photonic imaging is an ideal modality for biomedical diagnostics since it relates directly to a physician's vision and offers highly attractive characteristics, including use of non-ionizing radiation which does not damage the tissue, high flexibility in contrast mechanisms, portability, small form factor and real-time image acquisition. Healthy and diseased tissues exhibit differences in several properties such as structural, compositional, metabolic, molecular and structural. Local or systemic administration of agents with specificity to cellular and subcellular tissue and disease biomarkers could alter the optical properties of healthy and diseased tissue in a different way resulting in visualization of lesions with high contrast with the background healthy tissues. Recent studies indicate that the use of externally administered fluorescent probes is a highly promising approach since fluorescence signals can provide high contrast. For example, engineered probes can be very sensitive and specific in cancer detection by targeting specific molecular features of carcinogenesis and tumor lesions.
The need to efficiently detect the signal from molecular probes led to the development of several imaging methods and technologies in the last decade. Nevertheless, imaging methods used in practice suffer from limitations related to a) performance and b) convenience in use especially in clinical environments.
The imaging performance in resolving superficial fluorescence activity can be compromised by three major parameters: spatial variation in tissue optical properties, depth of the fluorescence activity and tissue auto-fluorescence. The dependence of signal intensity, e.g., fluorescence, on these parameters can limit both the contrast and the overall accuracy of uncorrected simple “photographic” or “video” methods. This can be better understood by considering, for example, that a dark, bloody area, significantly attenuates light intensity over a less absorbing region, an effect that can lead to false negatives. Similarly a non-absorbing area may show as probe rich compared to a dark region even at very moderate amounts of molecular probe. This can lead to false positives. Similar false positives or false negatives can be also generated as a function of the depth of the fluorescence lesion since light intensity non-linearly and strongly attenuates as a function of depth, i.e., light propagation in tissue. Therefore, unless one corrects for the variation in fluorescence signal intensity due to the variation of optical properties, variation of depth or auto-fluorescence raw images of tissue can be inaccurate or contain undesired artifacts. These effects have been noted in the past (e.g., see Ntziachristos et. al. Nature Biotechnology 2005; 23:313-320).
Systems that utilize imaging at multiple wavelengths have been developed to differentiate auto-fluorescence from a fluorochrome of interest. Similarly, variation of the intensity due to tissue optical properties and depth is typically corrected in tomographic systems.
On the other hand, further systems that show the potential to overcome the abovementioned limitations in performance are not suitable for clinical use due to poor functionality characteristics. For example, scanning multispectral systems can provide high spectral resolution but require time for scanning and therefore are not suitable for moving objects, i.e., real-time imaging operation. Therefore, they are not suitable for use on tissues moving due to breathing or heartbeat. Moreover, information generated by the images is not provided in real time and therefore such methods are impractical for scanning large tissue areas for lesions, zoom and focus on suspicious areas during examination and, last but not least, cannot be used for interventional procedures such as real time surgical guidance for lesion excision.
Overall, currently no medical photonic imaging system exists that accounts for the effects of light propagation and interaction with tissue in real-time to lead to accurate clinical imaging systems, for example, intra-operative imaging systems.
Tissue lesions, e.g., cancer, exhibit alterations in the tissue molecular, structural, functional and compositional characteristics. The use of targeting probes, e.g., molecular probes, has the potential to provide significant contrast between healthy and diseased tissue. Especially, with recent advances in genomics, proteomics and nanotechnology, new probes conjugated with appropriate optical markers, e.g., a fluorescent molecule or a photo-absorbing nano-particle, enable easier and more accurate detection of tissue structural, functional and compositional properties which could lead to non-invasive in vivo diagnostics. Ideally, an imaging modality able to capture those differences in optical signals and thereby detect and identify tissue lesions in real time could significantly increase our diagnostic, real-time guidance and interventional imaging capabilities.
Although several experimental methods have proven the potential of this approach, none of them exhibits sufficient performance for clinical use. The main limitations are: due to high complexity and inhomogeneity of biological tissues, photons undergo multiple and complex interactions with the tissue resulting in alterations to the measured signal. Correction of the measured signals requires a complex model that contains aspects of the tissue optical properties and/or geometrical characteristics. Reliable measurement of tissue optical properties requires fast acquisition and processing of a large amount of information. Existing imaging methods and technologies are limited as to the amount of information they can capture and correction they can offer.
Clinical applications such as surgical guidance require real time diagnostic or pathology feedback. In other words, signal capturing, processing and rendering of diagnostic result should be done in real time. Existing methods are limited by a tradeoff between analytical capabilities and speed.
US 2008/0312540 A1 discloses a system and method providing normalized fluorescence epi-illumination images and normalized fluorescence transillumination images for medical imaging. Normalization is obtained by combining an intrinsic image, like, e.g., a reflection image, and an emitted light image, like, e.g., a fluorescence image, collected at the sample. This conventional technique has limitations in practical applications, in particular due to the time needed for collecting images with multiple spectral ranges using changing optical filters or filter wheels, the duration of image data processing and a limited image quality. Furthermore, this technique has a restricted capability of providing diagnostic images since it only partially accounts for optical property changes, i.e., it accounts for absorption changes but not scattering changes.
It could therefore be helpful to provide an improved imaging device, in particular for multi-parametric real-time medical imaging, capable of avoiding disadvantages of conventional techniques. Furthermore, it could be helpful to provide an improved imaging method, in particular for collecting and providing photonic images for biomedical imaging with improved accuracy, being capable of avoiding disadvantages of conventional techniques.